This invention relates in general to artificial hearts and more particularly to an artificial heart system that will respond to varying physiological demand and includes mechanisms accommodating the actual flow imbalance between pulmonary and systemic circulations.
Over the last several years progress in developing a permanent artificial heart for implantation in a patient as a substitute for a failed natural heart has been steady. Initial clinical application of total artificial heart as a bridge to transplantation was done in 1969. This has been followed by several additional cases at various institutions. In 1982 the first pneumatically driven tethered artificial heart intended for permanent replacement was implanted. Among the issues that need to be addressed in an untethered artificial heart system are control strategies that respond to varying physiological demand, and mechanisms for accommodating the natural flow imbalance between the pulmonary and systemic circulations. The left-right cardiac output differences have been well established. Normally this difference appears to be ten to fifteen percent with the left side flow always greater than the right side flow. Artificial heart systems must account for this inherent physiological characteristic.
For externally actuated pneumatic systems the external drive system can be set to accommodate this flow difference. In permanent systems, however, this flow difference compensation has to be considered in conjunction with the system compliance and control. In the prior art two approaches, control valve regurgitation and a gas compliance chamber have been used in experimental systems. In particular a controlled outflow valve regurgitation in the artificial right heart has been employed. (Lioi, A. P.; Kolff, W. D.; Olsen, D. B.; Crump, K.; Isaacson, M. S.; and Nielson, S. D.; "Physiological Control of Electric Total Artificial Hearts", in Devices and Contractors Branch Contractors Meeting 1985, Program and Abstracts, Dec. 1985, 89.) In this approach the left and right sides of the heart are pumped alternately by a reversing hydraulic pump. A deliberate outflow leakage designed into the right pump is intended to accommodate the flow difference and obviate the need for a compensating chamber. However the regurgitant flow is only a function of the square root of the difference between the pulmonary diastolic pressure and the right atrial pressure. This results in a near constant compensating flow which may well be inadequate to accommodate time varying flow differences. Also, changes in the orifice size over the long duration can cause this flow imbalance to drift from the preset value.
A second prior art approach has employed a gas compliance chamber (with its problems of gas composition and pressure changes) and passive filling to accommodate the flow difference in conjunction with a stroke-time division scheme. (Rosenberg, G.; Snyder, J.; Landis, D. L.; Geselowitz, D. B.; Donachy, J. H.; and Pierce, W. S., "An Electric Motor-Driven Total Artificial Heart: Seven Months Survival In The Calf", Trans Am Soc Artif Intern Organ, 15, 69, 1984.)
In addition to the left-right balance problem, the prior art has actively worked on a control system for controlling the artificial heart. Externally actuated pneumatic systems have commonly been operated under preset drive parameters: drive pressure, vent pressure, beat rate, and systolic/diastolic ratio.